Protecting electrical apparatus with gas generating compounds

ABSTRACT

Electrical apparatus cooled with a stream of hydrogen gas is protected from overheating by applying a coating to a portion of the apparatus which is exposed to the gas stream. The coating contains a compound which decomposes between 80° and 200° C. to produce a gas detectable by a monitor. The monitor detects gases heavier than hydrogen in the gas stream and sounds an alarm when they are present.

This application is a division of application Ser. No. 751,403 filedDec. 16, 1976, now U.S. Pat. No. 4,149,161.

PRIOR ART

S. C. Browne, in a book published by the Massachusetts Institute ofTechnology Press, 1966, titled "Basic Data of Plasma Physics," disclosedthat the recombination rate of hydrogen ions will be slower if anothergas is present.

CROSS-REFERENCE TO RELATED APPLICATIONS

A number of the compounds used in this invention are described in thefollowing applications:

Ser. No. 645,164, filed Dec. 30, 1975, by J. D. B. Smith et al., titled"Sulfonic Acid Composition for Forming Thermoparticulating Coating," nowU.S. Pat. No. 4,046,733.

Ser. No. 568,224, filed Apr. 15, 1975, by J. D. B. Smith et al., titled"Diazonium Salt Composition for Forming Thermoparticulating Coating,"now U.S. Pat. No. 3,979,353.

Ser. No. 726,040, filed Sept. 23, 1976, now abandoned, by J. D. B. Smithet al., titled "N-thiophthalimide Composition for FormingThermoparticulating Coating."

Ser. No. 568,221, filed Apr. 15, 1975, by J. D. B. Smith et al., titled"Metal Acetyl Acetonate Composition for Forming ThermoparticulatingCoating," now U.S. Pat. No. 3,973,439.

Ser. No. 634,217, filed Nov. 21, 1975, by J. D. B. Smith et al., titled"Metal Dithiocarbamate Composition for Forming ThermoparticulatingCoating," now U.S. Pat. No. 4,056,006.

Ser. No. 699,561, filed June 24, 1976, by J. D. B. Smith et al., titled"Organo-Sulfur Composition for Forming Thermoparticulating Coating," nowU.S. Pat. No. 4,100,327.

Ser. No. 726,039, filed Sept. 23, 1976, by J. D. B. Smith et al., titled"Urea Composition for Forming Thermoparticulating Coating," now U.S.Pat. No. 4,108,001.

Ser. No. 702,257, filed July 2, 1976, by J. F. Meier et al., titled"Cyanoethylated Composition for Forming Thermoparticulating Coating,"now U.S. Pat. No. 4,168,625.

Ser. No. 568,222, filed Apr. 15, 1975, by J. D. B. Smith et al., titled"Blocked Isocyanate Composition for Forming ThermoparticulatingCoating," now U.S. Pat. No. 4,056,005.

BACKGROUND OF THE INVENTION

Electrical apparatus, such as motors and turbine generators,occasionally overheat due to shorts or other malfunctions. The longerthe overheating continues the more damage is done to the apparatus. Amalfunction detected immediately may mean only a quick repair but if theoverheating continues, the entire machine may be damaged.

Large rotating electrical apparatus is usually cooled with a hydrogengas stream. The organic compounds in the apparatus are first to beaffected by the overheating and they decompose to form particles whichenter the gas stream. Monitors then detect particles in the gas streamand sound a warning or shut down the apparatus when too many particlesare detected.

As the hereinbefore cited cross-referenced patent applications disclose,special coatings may be applied to the apparatus which decompose andform detectable particles at a lower temperature than the usual organiccompounds found in the apparatus. If an ion chamber monitor is used todetect particles in the gas stream, these coatings would thermallydegrade to produce particles (i.e., thermoparticulate) which results ina decrease in the current in the monitor.

SUMMARY OF THE INVENTION

We have discovered that certain compounds, instead of decreasing thecurrent in an ion chamber when they thermoparticulate, will actuallyincrease it. We have further found that this effect is due to theemission of a gas during or prior to thermoparticulation, rather than toa particle as is the case when the current is decreased.

This unusual effect is very useful in a generator because it provides aunique signal which can immediately tell the observer which area of thegenerator is being overheated.

Because the effect is caused by a gas and not by a particle, a signal ismore certain because gases do not deposit inside the generator asparticles tend to, but rather are carried ultimately to the monitor.This lack of deposition for gases also results in a much longer signalthan particles produce, so that a signal is less likely to be regardedas a temporary aberation.

DESCRIPTION OF THE INVENTION

FIG. 1 is a graph showing the relationship between the ratio of hydrogento carbon dioxide in a test gas to the current in an ion chambermonitor.

FIG. 2 is a diagram illustrating the method of this invention. In FIG.2, a coating 1 on a portion 2 of generator 3 becomes hot and releasescarbon dioxide into the cooling hydrogen stream 4 of generator 3. Aportion 5 of hydrogen stream 4 is diverted into monitor 6 then back togenerator 3. The monitor contains electrodes 7 and 8 which ionize thehydrogen gas into H- and H₂ + ions causing a current to flow betweenelectrodes 7 and 8. The current is sent via line 9 to recorder 10 andalarm 11. When carbon dioxide appears between electrodes 7 and 8 it isionized to CO₂ + and increases the current flowing between the electodesand to recorder 10 and alarm 11. The increased current is recorded byrecorder 10 and sets off alarm 11.

In an ion chamber monitor a radiation source disassociates hydrogen fromthe generator gas stream into H⁺ and H⁻ ions which are collected,producing an electric current. In the absence of particles, almost allthe ions are collected, resulting in a maximum output current of amagnitude determined by the strength of the radiation source, theionization properties of the gas stream, and the flow rate. Whenparticles are present in the gas stream, a combination between ion andparticle occurs. Because the particles are much larger than the ions,the mobility of the resultant charged particle is less, and fewer arecollected in the ion chamber. The result is a decrease in the outputcurrent of the ion chamber. The mode of action of all sacrificialcoatings developed to data has been to decrease the output current ofthe instrument.

In certain instances, however, the output ion current of the monitor canbe increased. The normal ion current, I_(o), with no particles presentis given by the equation: ##EQU1## where Q=flow rate

e=electronic charge

q=rate of formation of ions, ion pairs/ml/sec

and a=self-recombination rate of ions.

From the above equation, it can be seen that if a (theself-recombination rate) decreases, the ion current will increase. Inthe case of hydrogen alone, a is the recombination rate for H₂ ⁺ withH⁻. Generally speaking, the recombination of H⁻ with any other species(e.g., CO₂ ⁺) would be slower. Hence, in the latter instance, if CO₂were present in the hydrogen stream, a would be smaller and I_(o) (theion current) larger. However, not only must this species be liberatedinto the hydrogen stream, but it must be liberated in sufficientquantities to affect the overall self-combination value (as the normalH₂ ⁺ /H⁻ recombination will also be present due to the stream ofhydrogen).

A verification of this hypothesis was achieved experimentally bydetermining the effect of hydrogen and carbon dioxide mixtures on theion current of an ion chamber monitor. The gases were premixed prior tobeing passed through the instrument. A constant flow rate of 6liters/minute was employed. The results are shown in the accompanyinggraph. In the first set of experiments, a reference line for hydrogenwas developed using a 100% hydrogen gas flow. It can be seen from theother two lines in the graph that gas mixtures richer in carbon dioxideincrease the ion current, i.e., 10% CO₂ has a greater effect than 5%CO₂.

The invention, of course, requires a monitor which is capable ofdetecting gases heavier than hydrogen. An ion chamber monitor, whichoperates by ionizing the hydrogen and measuring the resulting electriccurrent, seems most suitable for the purpose of this invention. It isalso necessary that a compound be placed in the generator at a locationexposed to the gas stream which will thermally degrade to produce a gasdetectable by the monitor.

In practicing the invention a composition is prepared of a positivesignal producing compound (PSPC) in a solution of a resinous carrier.The PSPC may be dispersed if it is insoluble in the solvent (e.g.,toluene) or it may be in solution if it is soluble in the solvent (e.g.,ethyl alcohol or diethyl ether). Dispersions are preferred as theyproduce much more particulation than do solutions. A particle size ofthe dispersed PSPC of about 25 to about 1000 microns is suitable.

The composition may be prepared by simply mixing the ingredients, but itis preferable to mix the drier, resinous carrier, and solvent first andthen add the PSPC to prevent the occlusion of the drier in the PSPC andthereby obtain a more homogeneous dispersion of the PSPC.

A suitable composition is a resinous carrier, about 20 to about 250 phr(parts by weight per hundred parts of resinous carrier) of the PSPC andabout 25 to about 75% (by weight based on the resinous carrier) of asolvent for the resinous carrier. If the amount of PSPC is less thanabout 20 phr, the quantity of particles given off during decompositionmay be too low to be detected by presently-existing detectors. However,the construction of more sensitive detectors would permit a lower amountof PSPC. If the amount of PSPC exceeds about 250 phr, the composition isthick, difficult to apply, and does not bond well. The preferred amountof PSPC, which generally gives the best results, is about 40 to about 60phr. If the amount of solvent is less than about 25%, the composition isgenerally too viscous to apply easily and if the amount of solvent isgreater than than about 75%, the composition is unnecessarily dilute andthe coating may be too thin to produce an adequate number of particlesduring decomposition, at least while the malfunction is highlylocalized. Best results are usually obtained with about 45 to about 55%solvent.

The composition also preferably contains about 0.1 to about 3 phr of adrier when the resinous carrier is an epoxy resin or similar resin, topromote its room temperature cure. Lead naphthenate or cobaltnaphthenate is preferred although stannous octoate, zinc stearate, etc.,could also be used. Resins such as polyesters may also require thepresence of an organic peroxide as is known in the art. Mixtures ofvarious resins, solvents, or driers are also contemplated.

The resinous carrier performs the function of bonding the PSPC to theapparatus since a coating of PSPC by itself does not adhere well,although some PSPC may be found which can be used without a solvent orresin. The resinous carrier should be compatible with the other resinsused in the apparatus and therefore it is usually advantageous to usethe same resin used elsewhere. The resinous carrier is curable at 60° C.and is preferably air dryable since it cannot be easily cured in placewith heat. Also, it should be stable after curing for several years at60° C. The resin must be unreactive with the PSPC for otherwise suitablethermoparticulation will not occur. The PSPC and the resin form amixture and the PSPC does not catalyze the cure of the resin. Epoxyresins are preferred as they are usually used elsewhere in theapparatus, but polyesters, silicone rubber, polystyrene, etc., couldalso be used.

The solvent for the resinous carrier depends on the particular resinouscarrier used. Toluene, xylene, benzene, methyl ethyl ketone, ethylalcohol, diethyl ether, acetone, cellosolve, etc., are common solventsthat may be used. Toluene is preferred as it is inexpensive anddissolves most resins.

The composition is applied to portions of the electrical apparatus whichare exposed to the gas stream. The coating does not function asinsulation and is usually applied on top of insulation, but it can alsobe applied to conductors. The application may be made by brushing,spraying, dipping, grease gun, troweling, or other techniques. Asuitable coating thickness (after drying) is about 1/16 to about 1/2inch. The dispersed particles of PSPC should not be covered withexcessive resinous carrier as that may prevent the decomposition gasesand particles from escaping into the gas stream. After evaporation ofthe solvent and room temperature cure of the resinous carrier, ifnecessary, the apparatus is ready to be operated. Whenthermoparticulation and the resulting positive signal occur, a sample ofthe gas stream can be collected and analyzed. Since PSPC's generallyproduce particles as well as the initial signal-producing gas, analysisof the sample can confirm the location of the overheating.

An effort was made to determine which gases are detectable by themonitor. Six compounds which had been found to increase the current inthe ion chamber monitor were heated in an atmosphere of 20 torr H₂ in aglass tube attached to the inlet system of a mass spectrometer. Thecompounds were each heated to the lower limit of their increased ioncurrent temperature range and an aliquot of gas was withdrawn foranalysis. The temperature was then raised to the temperature of maximumsignal and a second aliquot was withdrawn and analyzed. The gasespresent were identified from the mass spectra, and quantitativeestimates were made for gases where calibration data were available. Theresults were summarized in the following table. The serial numbers referto the hereinbefore cited patent applications.

    __________________________________________________________________________                              Temp.   Mole %                                      Compound                  °C.                                                                        H.sub.2                                                                           CO.sub.2                                                                           Other*                                 __________________________________________________________________________    Zinc Acetylacetonate       88 58  --   42 2,4-pentanedione                    (Serial No. 568,221)                      + H.sub.2 O                                                   133 49  --   51 2,4-pentanedione                                                              H.sub.2 O                           Methenamine mandelate      99 100 TRACE                                                                              --                                      ##STR1##                 123 87  12   1  Benzaldehyde H.sub.2 O Unidentif                                              ied                                 Toluene diisocyanate-     136 91  TRACE                                                                              9  Toluene                             diethylamine                              Unidentified                        (Serial No. 568,222)      162 85   5   10 Toluene                                                                       Unidentified                        Ethylmethyl ketoxime-     125 88   7   5  Benzene + CO                        phenylisocyanate                                                              (Serial No. 568,222)      166 51  31   18 Benzene +                                                                     Unidentified                        Selenium dimethyl-        151 95  --   5  CS.sub.2                            dithiocarbamate                                                               (Serial No. 634,217)      188 64  --   36 CS.sub.2                            Hexamethylene diisocyanate                                                                              134 86  10   4  Unidentified                        with thiophenol block                                                         (Serial No. 568,222)      162 82  15   3  Unidentified                                                  192 55  45   --                                     __________________________________________________________________________     *Mole percentage determined by difference from 100%                      

From the above table it can be seen that all of the compounds dogenerate appreciable quantities of gases at the temperature range ofincreased ion current. For example, zinc acetylacetonate generatesapproximately 50 mole % of 2,4-pentanedione at approximately 130° C.

The self-recombination rate, a, would be lower in every instance whenthe recombining ions are H⁻ with CO₂ ⁺ (or CS₂ ion, or pentanedione ion,or benzene ion or toluene ion). Hence, it is likely that the increasedion current is due to the rapid evolution of these other gases into thehydrogen gas stream.

The gas that is produced should be free, rather than in an aerosol form,so that it can combine with the hydrogen ions. Also, of course, the gasmust be ionizable, which excludes certain fluorinated gases. Of thegases found to be operable sulfur dioxide is preferred because it is agas which is usually not present in a generator and therefore itspresence is a clear indication of over-heating. At least 1 mole of gasshould preferably be emitted per mole of compound for otherwise too muchcompound must be used.

An extensive search was made which resulted in identifying a number ofcompounds which produce an increased current in an ion chamber. Thefollowing example describes the preparation and testing of compositionscontaining these compounds.

EXAMPLE 1

The following composition was prepared using various PSPC:

    ______________________________________                                                             Parts by Weight                                          ______________________________________                                        PSPC                   100                                                    Epoxy resin, 50% solids in                                                    toluene, made from 200 pbw                                                    (parts by weight) linseed                                                     fatty acids, 200 pbw styrene,                                                 and 300 pbw diglycidyl ether                                                  of Bisphenol A, sold by                                                       Westinghouse Electric Cor-                                                    poration as "B-276" Varnish                                                   (See Example I of U.S. Pat.                                                   2,909,497 for detailed des-                                                   cription)              100                                                    6% solution in low boiling                                                    hydrocarbons of cobalt                                                        naphthenate            1.0                                                    24% solution in low boiling                                                   hydrocarbons of lead naphthenate                                                                     0.25                                                   ______________________________________                                    

The cobalt and lead naphthenate solutions were added to the epoxy resinprior to the addition of the PSPC.

Samples were prepared by brushing the above composition onto 3 inch by 1inch aluminum sheets 1/16 to 1/4 inches thick. The samples were driedovernight at 60° C. to form coatings 1/4 inches thick, then placed in aforced-air oven at 60° or 80° C. for various periods to determine ifthey were stable and would function after aging.

The samples were placed one at a time in a stainless steel boat within a1 inch o.d. stainless steel tube. Hydrogen was passed over the samplesat flow rates of 6 l/min. A phase-controlled temperature regulator andprogrammer controlled the temperature in the boat and the temperature inthe boat was measured by mounting a hot junction chromel-alumelthermocouple within a small hole in the boat. The output of thethermocouple and the detector were monitored on a two-pen potentiostaticrecorder. A 6° C./min. heating rate was maintained in each experimentafter the insertion of the sample in the boat.

The temperature at which the original amplified ion current increasedfrom 0.8 mA (to 1.0 mA and above) and the temperature at which it laterdecreased were noted; these two temperatures enable an "increased ioncurrent" range to be recorded for each sample. The following table givesthe results. Serial numbers refer to the hereinbefore cited patentapplications.

    __________________________________________________________________________                                     Increased Ion Current Region                                       Aging Conditions                                                                         Temperature                                                                          Duration                              Organic Compound used in Coating                                                                    (forced air oven)                                                                        Range (°C.)                                                                   of Signal (min)                       __________________________________________________________________________    Zinc acetylacetonate  8 days at 60° C.                                                                  88-93  5                                     Zinc acetylacetonate  29 days at 60° C.                                                                 84-91  5                                     Zinc acetylacetonate  46 days at 60° C.                                                                  95-102                                                                              5                                     Methenamine mandelate 44 days at 60° C.                                                                 100-118                                                                              8                                     Toluene diisocyanate-diethylamine                                                                   20 days at 60° C.                                                                 134-155                                                                              15                                    Toluene diisocyanate-diethylamine                                                                   8 days at 80° C.                                                                  134-158                                                                              5                                     Ethylmethylketoxime-phenylisocyanate                                                                1 day at 60° C.                                                                   137-169                                                                              10                                    Selenium dimethyldithiocarbamate                                                                    8 days at 60° C.                                                                  160-170                                                                              10                                    Selenium dimethyldithiocarbamate                                                                    15 days at 80° C.                                                                 161-175                                                                              12                                    Hexamethylene diisocyanate-thiophenol                                                               60 days at 60° C.                                                                  143-150 &                                                                           5                                                                      160-195                                                                              25                                    N-(cyclohexylthio)-phthalimide (Ser. No. 726,040)                                                   31/2 months at 60° C.                                                             160-177                                                                              12                                    N-(cyclohexylthio)-phthalimide (Ser. No. 726,040)                                                   31/2 months at 80° C.                                                             167-182                                                                              10                                    N-(phenylthio)-phthalimide (Ser. No. 726,040)                                                       31/2 months at 60° C.                                                             161-185                                                                              16                                    N-(phenylthio)-phthalimide (Ser. No. 726,040)                                                       31/2 months at 80° C.                                                             150-175                                                                              20                                    __________________________________________________________________________

The above table shows that some coatings are capable of initiatingincreased ion current at temperatures as low as 84° C. (zincacetylacetonate) and others as high as 161° C. (seleniumdimethyldithiocarbomate). The entire temperature range covered by thesecoatings is 84°-195° C. It should be noted that the duration of thesignal from hexamethylene diisocyanate-thiophenol was very long(approximately 30 minutes). All of these coatings produced low ioncurrents when heated to higher temperatures.

EXAMPLE 2

Two compositions were prepared and tested as in Example 1, after agingin nitrogen. The following table gives the results. In the table, thefirst temperature given in the organothermoparticulation range is thetemperature at which a negative signal (due to particles) was receivedand the second temperature is the temperature at which the current inthe ion chamber monitor dropped to half its normal value.

    __________________________________________________________________________                                Temperature of                                                                         Organothermopar-                                                     Positive Signal                                                                        ticulation Range                         Compound         Aging Condition                                                                          (°C.)                                                                           (°C.)                             __________________________________________________________________________    Morpholine p-toluene Sulfonate                                                                 120 days at 80° C.                                                                169      172-176                                                   91/2 months at 80° C.                                                             168      174-179                                                   11/2 years at 100° C.                                                             166      172-182                                  n-propylamine p-toluene sulfonate                                                              2 months at 80° C.                                                                170      173-175                                                   1 year at 100° C.                                                                 169      173-180                                  __________________________________________________________________________

The above table shows that the positive signal occurs at a lowertemperature than the negative signal and therefore provides an earlierwarning.

EXAMPLE 3

Compositions were prepared and tested as in Example 1. The followingtable lists the various compounds tested and the temperature at which apositive signal occurred. Serial numbers refer to theherein-before-cited patent applications.

    ______________________________________                                                                 Positive Signal                                                               Temperature                                          Compound                 (°C.)                                         ______________________________________                                        Morpholine-2-naphthalene (S.N. 645,164)                                                                143                                                  n-butylamine benzene sulfonate (S.N. 645,164)                                                          184                                                  2-(morpholino-thio)benzothiazole                                                                       147                                                  (S.N. 699,561)                                                                n-tertia-butyl-2-benzothiazole                                                                         110                                                  sulfenamide (S.N. 699,561)                                                    tetramethylthiuram monosulfide                                                                         140                                                  (S.N. 726,039)                                                                1,1,3,3-tetramethyl-2-thiourea                                                                         130                                                  (S.N. 726,039)                                                                Hydroxyurea (S.N. 726,039)                                                                             143                                                  Toluene diisocyanate/dimethylamine                                                                     124                                                  block isocyanate (S.N. 568,222)                                               β-napthol/acrylonitrile cyano-                                                                    136                                                  ethylated product (S.N. 702,258)                                              Benzyldimethylamino-p-toluene                                                                          173                                                  sulfonate (S.N. 645,164)                                                      n-propylamine-p-toluene sulfonate                                                                      165                                                  (S.N. 645,164)                                                                t-butylamine-p-toluene sulfonate                                                                       174                                                  (S.N. 645,164)                                                                Manganese (II) acetylacetonate                                                                         153                                                  (S.N. 568,221)                                                                N-oxydiethylene-2-benzothiazole sulfenamide                                                            127                                                  (S.N. 699,561)                                                                Hexamethylene diisocyanate/thiophenol blocked                                                          143                                                  isocyanate (S.N. 568,222)                                                     Phenyl isocyanate/butyglycidyl ether blocked                                                           137                                                  isocyanate (S.N. 568,222)                                                     Hexamethylene diisocyanate/dimethylamine                                                               134                                                  blocked isocyanate (S.N. 568,222)                                             Formamidine sulfinic acid                                                                              170                                                  Cadmium diethyldithiocarbamate                                                                         187                                                  (S.N. 634,217)                                                                Selenium diethyldithiocarbamate                                                                        156, 172                                             (S.N. 634,217)                                                                Morpholine-p-toluene sulfonate                                                                         168                                                  (S.N. 645,164)                                                                p-diethylamino benzene diazonium                                                                       147                                                  fluoroborate                                                                  t-butylurea (S.N. 726,039)                                                                              85                                                  Tellurium diethyldithiocarbamate                                                                       177                                                  (S.N. 634,217)                                                                ______________________________________                                    

We claim:
 1. A composition comprising a compound selected from the groupconsisting of formamidine sulfinic acid, methenamine mandelate, andmixtures thereof, and a solution of a resinous carrier curable at 60° C.stable when cured, and unreactive with said compound.
 2. A coatingcomprising a solid layer of a cured resinous carrier containing acompound selected from the group consisting of formamidine sulfinicacid, methenamine mandelate, and mixtures thereof.
 3. A compositionaccording to claim 1 wherein said compound is formamidine sulfinic acid.4. A composition according to claim 1 wherein said compound ismethenamine mandelate.
 5. A composition according to claim 1 whereinsaid composition comprises about 20 to about 250 phr of said compoundand about 25 to about 75% of a solvent for said resinous carrier.
 6. Acomposition according to claim 5 wherein the concentration of saidcompound is about 40 to about 60 phr and the concentration of saidsolvent is about 45 to about 55%.
 7. A composition according to claim 1wherein said resinous carrier is an epoxy resin.
 8. A compositionaccording to claim 7 wherein said composition includes about 0.1 toabout 3 phr of a drier for said epoxy resin.
 9. A composition accordingto claim 8 wherein said drier is lead or cobalt naphthenate.
 10. Acomposition according to claim 1 wherein said resinous carrier isair-dryable.
 11. A coating according to claim 2 which is about 1/16 toabout 1/2 inch thick.